Tube materials for steel tubes to be used under high-temperature environments such as boilers for thermal power generation plants, chemical industries, nuclear power plants, and next-generation fast breeder reactors are required to have excellent strength and corrosion resistance at high temperatures. Among them, a double-walled tube which is configured such that one of the walls is made of a material having an excellent corrosion resistance and the other is made of a material having excellent high-temperature strength is used as needed.
For example, Patent Literature 1 describes a double-walled tube formed by hot rolling a multi-layer billet in which alloy powder containing Cr and Ni as principal elements is packed on the outer surface of a billet made of Fe—Ni—Cr austenitic heat-resistant steel with a Cr content of not more than 30%. Further, Patent Literature 2 describes a double-walled tube in which one of the walls is made up of an austenitic stainless steel tube containing not less than 30% of Cr, and the other is made up of an austenitic stainless steel tube containing not less than 25% of Cr.
These double-walled tubes are of a type that is free of an interface gap, in which tubes of dissimilar metals having different properties, such as a corrosion resistance material and a high-strength material, are combined and the outer wall tube and the inner wall tube are physically or metallurgically bonded to each other (that is, the inner surface of outer tube and the outer surface of inner tube are brought into contact via metal surfaces, or in a metallic bond to each other).
However, since in a metallurgically bonded double-walled tube without the gap, the metal surfaces of the outer wall tube and the inner wall tube are physically or metallurgically bonded to each other, and it is not possible to prevent a through-wall leakage (a leakage which occurs as a result of cracking propagating through the double-walled tube) which can occur, even if in a worst case scenario, in a heat transfer tube (double-walled tube) in which liquid Na as coolant is circulated, for example, in a steam generator (SG) of a fast breeder reactor (hereafter referred to as “FBR”).
If in any chance a through-wall leakage occurs in a double-walled tube to be used in a steam generator tube of a next-generation fast breeder reactor (FBR), reaction and explosion will occur as the result of contact between liquid Na and water (vapor), which is very dangerous. Therefore, the prevention of through-wall leakage is the most important issue in a fast breeder reactor (FBR).
In contrast, there has been developed a double-walled tube in which metal surfaces are not physically or metallurgically bonded to each other and the interface gap is provided therebetween. A major feature of the gap of a double-walled tube is that even if cracking occurs either in the outer wall tube or the inner wall tube, the gap will deter the propagation of the cracking, thereby preventing an immediate occurrence of a through-wall leakage. Further, there has been developed a technique in which an inert gas such as helium is flowed by utilizing the interface gap of the double-walled tube, and even if, by any chance, either of the inner wall tube or the outer wall tube fails, the failure is promptly detected, thereby preventing a through-wall leakage accident.
For example, Patent Literature 1 proposes a double-walled tube including a braided mesh wire, in which a braided mesh wire is inserted between the inner wall and outer wall tubes, the braided mesh wire being formed by bundling and weaving an element wire (100 μm) made of the same material as that of the tube. By allowing inert gas such as helium to flow through the interface gap provided in the double-walled tube, it is possible to promptly detect the fracture of the inner wall tube or the outer wall tube thereby preventing a through-wall leakage.
Further, Non Patent Literature 2 describes a double-walled tube with a groove(s), in which a groove processing for detecting a leakage is performed on the inner surface of the outer wall tube.
However, the production of such a double-walled tube with the interface gap according to such prior art requires the repetition of very special, intricate and elaborate processing. Thus, in addition to considerable difficulty in production, the prior art suffers from complexities thereof so that a great deal of man-hours are required and a large number of defects may generate, and therefore it is not suitable for mass production and poor in economic efficiency. Further, since there is contact between metal surfaces, there still remains a risk that cracking may propagate and a leakage going through the double-walled tube may occur. Further, it is difficult in reality to produce a longer-length double-walled tube of 35 m or so in length which is envisioned in next-generation FBRs.
As another prior art, Patent Literature 3 describes a cooling double-walled tube for a stave cooler which is installed in the inside of furnace wall of a blast furnace, and the like. This double-walled tube is produced by placing a foreign layer of oxides etc. in the interface between the inner and outer wall tubes which are made of carbon steel, and subjecting them to a drawing process. However, such a double-walled tube cannot be used for heat exchanger tubes since the oxides etc. placed in the interface circumvent and shield the heat transfer between the outer and inner wall tubes. Besides, the tube cannot be used for the above, since the tube material is limited to carbon steel, the strength standard for a high-temperature and pressure-resistant member which is used under environments of not less than 450° C. is not satisfied and corrosion resistance is also insufficient.